Problem: $ E = \left[\begin{array}{rr}-2 & 5 \\ 0 & 5\end{array}\right]$ $ D = \left[\begin{array}{rrr}3 & 2 & 1 \\ -1 & 2 & -1\end{array}\right]$ What is $ E D$ ?
Answer: Because $ E$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ E D = \left[\begin{array}{rr}{-2} & {5} \\ {0} & {5}\end{array}\right] \left[\begin{array}{rrr}{3} & \color{#DF0030}{2} & \color{#9D38BD}{1} \\ {-1} & \color{#DF0030}{2} & \color{#9D38BD}{-1}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ E$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ E$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ E$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{3}+{5}\cdot{-1} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ E$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{3}+{5}\cdot{-1} & ? & ? \\ {0}\cdot{3}+{5}\cdot{-1} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ E$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{3}+{5}\cdot{-1} & {-2}\cdot\color{#DF0030}{2}+{5}\cdot\color{#DF0030}{2} & ? \\ {0}\cdot{3}+{5}\cdot{-1} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{-2}\cdot{3}+{5}\cdot{-1} & {-2}\cdot\color{#DF0030}{2}+{5}\cdot\color{#DF0030}{2} & {-2}\cdot\color{#9D38BD}{1}+{5}\cdot\color{#9D38BD}{-1} \\ {0}\cdot{3}+{5}\cdot{-1} & {0}\cdot\color{#DF0030}{2}+{5}\cdot\color{#DF0030}{2} & {0}\cdot\color{#9D38BD}{1}+{5}\cdot\color{#9D38BD}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}-11 & 6 & -7 \\ -5 & 10 & -5\end{array}\right] $